The present invention relates to a method of monitoring the internal impedance of an accumulator battery in an uninterruptible power supply, and to an uninterruptible power supply.
In uninterruptible power supplies (UPS) for computers and other apparatus that are sensitive to power interruptions and the like, the reserve energy source in general consists of a series of lead acid cells that, in a case of power interruption, can be rapidly connected electronically via an inverter and thereby AC current can be supplied to the system connected to the UPS. The purpose of the UPS is partly to bridge shorter power interruptions, partly to signal the data system in cases of mains interruptions of longer duration, so the computer can be shut down automatically in a controlled manner without damage, e.g., lost files, before the limited energy stored in the battery is completely exhausted.
In certain known UPSs, the condition and charging of the battery is hardly supervised at all, but is controlled by means of continuous charging voltage (float charge) that must be sufficiently high to charge all cells in the battery but not so high as to overcharge any of the cells. Undercharging of the battery may result in sulphatation (PbSO.sub.4) of the electrode plates in the cells. Such sulphate build-up is a normal and reversible discharge process in a lead acid battery, but in a continuous undercharge condition also non-recoverable sulphate crystals degrading the capacity of the battery are created. Overcharging in turn may result in drying out of the electrolyte of the battery and cause elevated corrosion of the electrode grids and associated conductors. Overcharging may further cause bridging phenomena finally resulting in short circuiting between the electrodes. Also overheating of individual battery cells may occur, particularly with several series connected battery cells, as variations in the individual voltages of the cells may appear.
Other known solutions utilize individual cell-voltage measurements, but the associated hardware and installations are expensive. Further, it is a known fact that potential battery failures are best detected during discharge, particularly near the point when the nominal capacity of the battery is nearly totally exhausted, i.e., where variations in the charged capacity are readily apparent. There are also applications in which deliberate discharge of the battery is performed for a certain period of time, but the discharge that is required to achieve a reliable estimate of the battery capacity must be weighed against the risk of power interruptions when the battery is in its discharged state. In critical applications, this risk is not acceptable.
It is also known that the internal impedance of a battery, expressed for instance as the value of an AC voltage applied over the battery divided by the AC current obtained in the battery, is an indicator of the general condition of the battery. The internal impedance of a battery increases with increasing sulphatation of the electrodes, corrosion of conductors, etc., and with increased drying out of electrolyte in the battery. The internal impedance of the battery therefore constitutes an important indicator of the performance of the battery. However, there is no known system available that would measure this impedance in a simple manner.